Recent anatomic and physiologic studies suggest that the primate ventral thalamic nucleus (human ventrocaudal nucleus - Vc) which receives innocuous mechanoreceptive input is also involved in nociception. Studies in animal models of chronic pain suggest that this same nucleus may be involved in some types of chronic pain. We propose to examine the role of human Vc in pain by studying thalamic single unit activity recorded as part of the physiologic localization required during stereotactic procedures for the treatment of pain and movement disorders. Initial studies will attempt to define nociceptive inputs to Vc by examining neuronal responses to application of quantitative somatosensory stimuli. In pain patients, spontaneous and evoked cellular activity will be studied in regions of Vc which represent parts of the body where the patient experiences pain. These studies will attempt to identify patterns of thalamic activity associated with human chronic pain. Finally, patterns of thalamic activity and psychophysical estimates of pain will be studied simultaneously. Patterns of thalamic activity will be interpreted in light of recent advances in thalamic physiology and so may identify intracellular mechanisms involved in thalamic activity related to chronic pain. These studies may lead to the development of new therapeutic strategies for the treatment of chronic pain.